1. Field of Invention
The invention concerns a method and device for positioning of valve or cover sheets relative to components of tube pieces.
2. Description of the Prior Art
Positioning of valve or cover sheets relative to components of tube pieces is important, especially in the production of sacks and bags. Such sacks, which are often prescribed for later filling with bulk products, for example, construction materials like cement, are generally produced by transporting a section of a tube across the tube axis, attaching the end areas, applying a valve strip, for example, gluing it on, then folding the parts of the applied end areas to close the sack end and finally applying bottom cover sheets to the bottoms so formed. If required, one of the two ends can be formed in a different way and it is often not essential to provide a valve on both ends. Bottom cover sheets can be dispensed with, if the strength of a bottom is already sufficient. Such a sack is later filled through the valve or a valve. However, for quality of the sacks often not only the correct position of the valve or cover sheet is important, but also the correct position of the printing, coatings and any glue application and also protrusions or grooves that mark later folding lines. Production of such sacks from tube pieces occurs in devices that are often referred to as bottom-laying devices.
To produce the described sacks the tube pieces are transported across their sack longitudinal axis and in the plane of the tube generally horizontally in a transport device, in which they are kept unmovable relative to the transport device. For this purpose the tube pieces are often clamped in the area of their two ends between the belts of so-called double conveyor belts. The valve sheet and the bottom cover sheet are transported by other transport devices to the tube pieces, in which double conveyors or also cylinders are provided for this purpose, on whose surfaces the valve or bottom cover sheets are transported. Such cylinders can be equipped with suction devices or pincers. At the beginning of the production process all transport devices must be adjusted and tuned to each other so that the valve sheet or bottom cover sheets are placed precisely at the target positions of the components of the tube pieces to be fastened there. In this case not only does the position of the sheet and strips in the transport devices play a role, but so do the transport speeds. For tuning and adjustment on devices according to the prior art random samples are taken at the beginning of production by the machine operator and measured in order to determine the deviations of the actual position from the target positions. The individual transport devices are then adjusted according to the determined deviations in order to minimize the deviations. Adjustment work is also necessary on the devices for printing, coating or embossing!A drawback here is that the adjustment process is time-consuming and the sacks are damaged or even destroyed for control.
DE 195 02 830 A1 proposes a test method for monitoring of deviations. During an unduly large deviation of an actual position from a target position the corresponding sack is later sorted out as reject.
During use of this procedure, however, it has been found in the past that the geometric product tolerances, i.e., the deviations of the actual position or the determined positions from the target positions often vary over a wide range and considerable rejects are therefore produced. In such cases the bottom-laying device can be readjusted, but production must be interrupted again for this purpose.
The task of the present invention is therefore to propose a method and device with which production tolerances can be further minimized without additional time loss and the percentage of rejections reduced.
The task is solved by a method and a device as described herein.
According to it images of the components of several tube pieces and/or units that act on the tube pieces or the components that are applied to the tube piece are initially recorded. These images can be recorded after different production steps during sack production. In this case, for example, each sack can be imaged or only every n-th sack (n>1). One or more images of a sack can be recorded, depending on which positions are to be determined. Thus, it can be useful to record images of the grooved tube pieces before the valve sheet is glued on. After gluing on of the valve sheet another image can be recorded.
In a second step the actual positions of the valve or cover sheets, imprints, coatings and/or embossings are determined relative to a reference point of the tube piece. This reference point is roughly the protruding triangle vertex that forms, when a flat-lying tube piece, which is conveyed across a tube axis but in the plane of the tube, is attached on its ends. The attached end then lies parallel to a plane lying perpendicular to the plane of the tube and orthogonal to the tube axis. The positions in the plane of the attached end are then preferably determined while the positions in the orthogonal direction to this plane have subordinate importance. The difference between the actual positions and the corresponding target positions is then determined in order to obtain the deviations from the target positions.
If, however, a cover sheet to be glued on is imaged on this feed cylinder but no reference point of the tube piece is imaged simultaneously, the actual position must be set with reference to a recorded reference point whose position in space is known. In order to be able to position the image with reference to the tube piece, the tube piece must trigger the recording when it passes a certain location.
A value by which the valve or cover sheet, imprints, coatings and/or embossings are shifted relative to the components of the tube pieces now follows from the calculated deviation. This can occur by movement of the units that carry out the mentioned attachments or introductions. As an alternative or in addition the devices that transport the tube pieces, for example conveyor belts, can also be moved. The transport devices can also be accelerated or braked when the determined deviation lies in the transport direction of the tube pieces.
The value that follows from the determined deviation can be the determined deviation itself.
The deviation is determined on a tube piece on which, however, an influence can no longer be exerted. The position changes therefore concern the following tube pieces. A position change, however, can lead to greater deviations for the following tube pieces, i.e., reverse the desired effect.
In an advantageous variant of the invention it is therefore proposed to determine the value that follows from the determined deviation from a number of deviations, in which an average is formed from this number. An average deviation is therefore calculated by average value formation over several recorded tube pieces. The position of the corresponding unit is then changed so that the corresponding actual position is corrected by the calculated average. This correction can also occur when this average lies within the product tolerance. Not only does position of the unit refer to the spatial position relative to the tube piece, but also the phase position relative to the transport device that transports the tube pieces.
The number of tube pieces over which the average is formed is advantageously the number of tube pieces that are transported during one revolution of a belt for transport of the tube pieces.
In another advantageous variant an average deviation is not calculated from the determined deviations but a deviation function is determined. This deviation function can be dependent on time. The function value of this function at a fixed time can then be the deviation value with reference to an individual sack or individual tube piece, but the deviation values from already measured or still to be measured sacks can also be considered (sliding average formation).
By means of the setup function not only can already determined deviations be reacted to, but also future deviations predicted. This is particularly advantageous, if periodic deviations occur in the bottom-laying devices, for example, synchronism deviations of the transport belts or drive motor. In an advantageous variant the function is also a periodic function. A correction of the actual positions can then also occur periodically by means of a control and regulation unit, in which the value that follows from the determined deviation and is used to adjust the positions is the function value of the function at a specific time.
Recording of images occurs with one or more cameras that can be mounted on the machine frame of the bottom-laying devices. These images recorded by the cameras are sent to an evaluation and computer unit, which determines from the recorded images the positions of the reference point and the actual positions, calculates the deviations from the target positions and determines the average values or functions. These average values are then sent to a control unit which drives the servomechanisms with which the positions of the imprints, coatings, embossings, valve sheets and/or cover sheets can be changed for the attachment or insertion units. These servomechanisms can be motor-adjustable differentials, which can be adjusted during running production so that the correction of the positions can occur without interrupting production. Such differentials, also referred to as compensation drives, are generally used in order to change the phase positions and/or rotational speeds of two components driven by a drive relative to each other. A number of units on a bottom-laying device are often driven by means of a signal drive, which drives a so-called bevel shaft, from which a torque for a unit is taken off.
The aforementioned method according to the invention can be repeatedly conducted in continuous fashion during sack production in order to avoid rejects as fully as possible.
Additional advantageous variants of the invention are apparent from the dependent claims connected to the independent.
A practical example of the invention follows from the description and drawing.